John Curl's Blowtorch preamplifier

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#1,603
Steve Eddy said:
Well, yes. It would be easier if DigiKey um... actually stocked that transformer. 😀 se
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Read back through this thread and, if you don't move many light years ahead through a combination of true electronics fundamentals tempered by seat of the pants experience, then you're missing a significant piece of the roadwork.

I was ready to nominate John for sainthood after tripping across this thread a couple of years ago, but then, I don't get out much.

If this thread was to take a less judgemental tone and just discuss things at face value, there's alot to learn. No need to simulate, just try this and report back as to what it sounds like...

Ah, but this is 2007 (question: isn't there a really cool high-tech approach to all this?) (Answer: just listen to what is out there in reproduced audio and there's your answer...). A DIY experimenter has a definite leg up.
 
#1,609
Well I think JC has been very helpful by giving a lot of information on the Blowtorch. So please try to appreciate this!

I don’t have the skills to make a copy of the Blowtorch, but have found some good information to a less complex project.

Thanks Mr. Curl for sharing some ideas and his knowledge.

Vogue
 
#1,610
Balanced has lower distortion, because it cancels any even harmonics and lowers the drive voltage on each leg to the power amp. Because I am using a Jenna Labs balanced, but unshielded interconnect between the preamp and the power amp, it is best that I use it in balanced mode to cancel any stray pickup from the air.
I am not too concerned with a little extra residual 2'nd harmonic, but it does embarrass me that I did not get it perfect in the first place. The circuit is completely balanced, but the active parts are always a little off, especially from N to P, and this adds extra even order distortion.
 
#1,611
PMA said:
Regarding shielding - I design measurement instruments for high power/high voltage testing laboratories and I am quite familiar with this issue. These instruments are influenced by both LF high B and very fast HF EM fields.

Al or Cu are great for shielding HF fields, and also for LF fields (magnetic fields!) in case they are thick enough.

Skin depth for Al:
50Hz ... 11.6mm, 1kHz .... 2.62mm, 10kHz .... 0.82mm

Skin depth for Cu:
50Hz ... 9.3mm, 1kHz ... 2.1mm, 10kHz ... 0.66mm

There is one great advantage of non-magnetic (eddy currents) shielding - no saturation. Iron saturates, depending on permeability. Mumetal or Giron are great for shielding of low intensity LF magnetic fields. But they saturate early (0.7T, 2T) and as soon you get into saturation you loose all advantages.

I have great experience with thick alluminium. I may show photos of casework parts milled from thick alluminium later.
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"Skin dept" ... could you explain ...

Planning on making a all alluminum chassis for my passive pre-amp (also almost airtight) - is 8 mm alluminium ok ... or should i try to find something even more heavy.

Vogue
 
#1,612
Hi Vogue,

skin depth is nicely described in Wiki:

http://en.wikipedia.org/wiki/Skin_depth

much better than I would be able to explain here.

Regarding 8mm Al, it is fine for your purpose. Do not forget to have all the mechanical parts of case electrically connected (there must be no anodization on connecting surfaces).

Also, see

http://en.wikipedia.org/wiki/Hysteresis

to understand saturation in iron.

Regards,
Pavel
 
#1,615
Back when I played around with these factors last time, I found that either 20mm of aluminum or 10mm of carbon brings you to the point of diminishing returns....And really, if you're willing to take on the trouble of making an 8mm thick box, you might as well go all the way and make it 20mm. It will not cost you much more and it will be the same amount of work.

Magura 🙂
 
#1,616
janneman said:
Question to you shielding experts:

I have access to stainless steel sheets of 2mm (0.079inch) and 3 mm thickness (0.12inch). How would that compare with aluminium as far as screening of an enclosure is concerned?

Jan Didden
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It depends which alloy you got there. I made some shielding for inductors of ferritic stainless steel with great results, but austenitic stainless steel will offer little screening.

Magura 🙂
 
#1,618
Magura said:
Back when I played around with these factors last time, I found that either 20mm of aluminum or 10mm of carbon brings you to the point of diminishing returns....And really, if you're willing to take on the trouble of making an 8mm thick box, you might as well go all the way and make it 20mm. It will not cost you much more and it will be the same amount of work.

Magura 🙂
Click to expand...

I really do use aluminium boxes (15 pcs now) for shielding of opto-electronic fibre coupled transient recorder. The transmitter is battery supplied, inside aluminium box made of 8mm milled plates. Dimensions are about 40 x 35 x 15 cm. Parts with step-milled profile to assure electrical contact, screwed by 40 screws. The environment is high-power/high-voltage testing plant with both strong mag. field (100kA/50Hz) and fast transient el. field.

Thick aluminium works better than 3mm iron. Iron saturates and has unpredictable HF properties. Thick aluminium was even better than 3mm iron covered by anodized Cu film.
 
#1,620
My plan is to use prefabricated aluminium tubes or profiles. I found some tube with a diameter of 216 mm and a “wall” thickness of 8 mm. I would the close up the chassis with some 8 mm. aluminium sheets front and back. The price could be kept at around 300 USD for the tube and the two sheets.

I know the design would be more or less like a nuclear bomb … but this should be very effective!

I have a question concerning the use of Teflon sheets for the RCA jacks in the Blowtorch – I assume it’s Teflon sheet. Why not use copper sheet … or even a aluminium sheet.

Vogue
 
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